1. Field of the Invention
This invention relates generally to a method of curing tires, and more particularly to such a method of curing tires characterized by holding the curing mold in a stably closed, locked condition during the curing process under purely mechanical force which in one aspect is especially adaptable for curing very large-sized or giant tires.
2. Description of the Prior Art
Large or small-sized tires generally used for cars, trucks or buses are conventionally produced by a composite unit known as a mechanical press which includes a semi-shaping unit and a curing unit, whereby the tires of such sizes are produced in an automatic production line system featuring such processes as semi-shaping, curing and removing the cured tires.
However, very large-sized or giant tires which are used for construction machines, super-heavy trucks or other special vehicles have not generally been in such demand as to require mass production, and therefore, the production of such very large-size or giant tires has not conventionally been placed upon an automatic production line system. Instead, the very large-sized or giant tires for the above-mentioned purposes are generally produced in the following manner. First, a "green" tire, or one which is semi-shaped substantially corresponding to the final shape but not yet cured, is produced in a semi-shaping unit. Then, the semi-shaped tire is placed in a mold unit composed of upper and lower mold halves, and the mold unit containing the semi-shaped tire is carried into a pot heater equipped with hydraulic ram means. In the pot heater, the upper and lower mold halves are pressed together by the hydraulic ram means and perhaps additionally, for security purposes, by means of bolt and nut arrangements with such sufficient force to overcome the strong expansion force exerted in the curing process by the tire-forming materials contained in the mold.
In the curing process, the mold unit is confined in an atmosphere of steam for heating purposes for as long as about 6 to 12 hours, while the inner surface of the tire is supported by a bladder into which hot water is introduced under pressure. After curing, the mold unit is taken out of the pot heater, and the cured tire, including the bladder therein, is removed from the upper and lower mold halves. Finally, the tire is again processed in the semi-shaping unit so as to remove the bladder therefrom. The series of processes described herein and conventionally used for curing very large-sized or giant tires are not performed automatically.
However, a remarkable increase in the demand for such very large-sized or giant tires in recent years has pushed forward the requirement that very large-sized or giant tires also be mass produced with the same high efficiency previously realized in the production of large and small-sized tires.
When very large-sized or giant tires are produced in the conventional method described above, there is a serious disadvantage, other than that the conventional method is not readily suitable for an automatic mass production system, which is that it sometimes is difficult to keep the mold halves containing the tire-forming materials in perfectly closed condition for a long period of time, such as between 6 and 12 hours, against the strong expansion force exerted by the materials. This is because, in the conventional method wherein the mold halves are pressed together by a hydraulic ram means, there is a danger that the pressing force of the ram may be unexpectedly lowered or lost due to a failure of the electric power or a damage in the electric system, or due to a leakage of hydraulic fluid in the sealing portions of the ram cylinder or valves.
If a lowering or loss of the ram hydraulic pressure occurs during the curing process, the tire-forming materials contained in the mold unit will leak out of the mold through the joining portion of the mold halves, whereby the product is degraded and is below standard such that it cannot be used. Such failure in the producing process of very large-sized or giant tires results in serious loss, since such tires are generally very expensive. To prevent accidental damage of the character described from occurring, attempts have been made to clamp the upper and lower mold halves by bolts and nuts, in addition to the hydraulic ram means, but this clamping work is heavy, difficult, and expensive, and nevertheless, it cannot perfectly accomplish the purpose of keeping the mold unit in a satisfactory tightly closed condition in the case of failure of the ram hydraulic pressure.
Further, there is the fear that operators are exposed to dangerous situations caused by the loosening of these bolts and nuts during the production process, and a still further disadvantage of the conventional method of employing hydraulic ram means is that it requires a number of pumps and valves, and therefore, the maintenance of the equipment is relatively expensive.